I'm beginning to think this is a myth. Four year holds DON'T speak perfectly. They are often making grammatical errors and pronunciation mistakes.

On the other hand, an adult who spoke a new language exclusively for four years intensively, without any distractions with a first language, and who mediated all relationships and daily living essentials in the new language (including an intense, loving interaction with a main teacher) would easily be fluent after 4 years and probably speak it better than an average four year old.